If you look at a typical flat-colored rubber-hose style appendage as in "a" it's rather ambiguous as to which way it is really bending.
But if you add that pant cuff just a little bit down the leg as in "b" or "c" you get a clue as to the intended direction of the knee.
Of course, there are lots of times that Mickey doesn't wear shorts, but then they have to work harder at drawing him with real articulated knees.
